Abstract Transition-age youth (TAY) from ages 18-26 are especially vulnerable to substance use, misuse, and substance use disorder (SUD). This developmental period, termed the ?age of instability and emerging adulthood includes tasks such as leaving home, entering college; identifying vocational goals; working for the first time; body and sexuality changes; coalescing with a peer group; and for some, aging out of foster care or state custody, which end at age 18. The emerging- adulthood period is prime for experimentation with substances and the development of SUD, and associated problems such as binge drinking, driving under the influence, accidents, fighting and violence, HIV, gang involvement, suicide and self-harm, and vulnerability to date-rape and other sexual assault. A major challenge for TAY is the capacity to regulate their emotions, given the hormone changes and emotional intensity of this stage of life. Indeed, substance use is often described as a short-term way to regulate emotion and there is a long-standing literature documenting the association between emotion regulation (ER) problems and SUD. The importance of ER is also grounded in the developmental literature in which ER is identified as a core skill that is critical to the successful transition to adulthood. The initial component of ER, accurate identification of emotion, is itself a major challenge, especially for people with SUD as they are often not aware of their feelings or confused about them due to the nature of the disorder as well as mental health disorders that often co-occur with SUD. In our phase 1 project we developed and evaluated a mobile app that uses exciting new technology (automated emotion detection) to help TAY better recognize their emotions in relation to SUD variables. We found highly positive results in our phase 1 feasibility / pilot study and are now proposing a phase 2 project to continue this work. In phase 2 we plan to enhance and continue to develop the app features and user experience based on end-user feedback and our expert consultant team. Phase 2 also includes a randomized controlled trial to evaluate the app among TAY with SUD. They will have six weeks to use the app in their natural environment, such as home, and we will conduct pre- and post-evaluation using validated instruments to compare outcomes between two conditions: the app condition and a control-app condition. The primary outcomes are substance use and emotion regulation. We also plan to evaluate end-user satisfaction with the app and have a final round of feedback at the end of phase 2 as well as explore app metrics. The app product we envision would have major public health and clinical impact. It could help improve TAY?s ability to become more aware of their emotions and understand them in relation to SUD via an exciting emotion-focused technology innovation. If the product is successful it could also be expanded to other populations. Our stellar team includes experts in SUD, TAY, clinical innovations, technology, and app development.